How do you measure height on Mars?

We use sea level to measure height on Earth. What is used on Mars? (or other planets for that matter)

Thousands of point elevation measurements from the laser altimeter onboard the Mars Global Surveyor spacecraft were processed to create this computer-generated view of the largest volcano yet known in our solar system, the 22-kilometre-high Olympus Mons (Source: NASA/MOLA Science Team/ O. de Goursac, Adrian Lark)

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I've been watching with interest the NASA program on Mars. I'm intrigued by the height of the terrain and how they quantify it. For example they indicate that a mountain will be five kilometres high. To what datum on Mars is this measured. We use sea level on Earth, what is used on Mars? (or other planets for that matter)—Tim

Since landing on Mars 12 months ago, NASA's Curiosity Rover has been on a journey to Mount Sharp, a five-kilometre-high peak in the middle of Gale Crater near the red planet's equator.

But what reference point do they use to measure the height of Mount Sharp? Is its peak five kilometres above the lowest point on Mars? Or is it the average diameter of the planet?

"A lot of people think it's just taking the highest and lowest points on the planet and finding the average, but it's not that simple."

"Because there's no sea level on Mars any more, zero altitude is defined as a specific atmospheric pressure of 610.5 Pascals, about six millibars," says O'Toole.

"This value was chosen because it's the triple point of water on Mars, where it can exist as gas, liquid or solid."

To measure the height of a mountain, or depth of a canyon or crater, scientists have relied on data collected by orbiting spacecraft.

"These days we usually bounce some kind of electromagnetic radiation off a planet's surface and typically radar is the classic tool for these measurements."

More recently, NASA's Mars Global Surveyor used a laser altimeter, which transmits infrared laser pulses towards the Martian surface at a rate of 10 times per second, to produce a highly detailed map of the surface.

Moon and Earth

O'Toole says a similar instrument was used by NASA's Lunar Reconnaissance Orbiter spacecraft to provide a precise topographic model of the Moon.

Because it has is no ocean or significant atmosphere, the zero elevation point of the Moon is the average diameter.

Back on Earth, mean sea level is used as the reference point for measuring elevation.

"Mean sea level is measured over time so you can average out the changes in sea levels due to waves, tides and storms," says O'Toole.

"Because there are lots of tide gauge stations in ports and harbours around the world, we can get a really accurate reading."

But, O'Toole says this level varies from ocean to ocean with substantial irregularities of up to two metres.

One reason for this is that the Earth isn't a perfect sphere. Its rotation causes it to bulge out at the equator and flatten slightly at the poles, resulting in a difference of about ten kilometres between the equatorial and polar circumference.

The oceans are also influenced by the gravitational pull of the Earth which is not constant across its surface. In some places, there is more mass under the Earth's surface, resulting in a slightly stronger gravitational pull towards the centre. These irregularities form very gentle but widespread "hills" and "valleys" across the ocean's surface.

"So as you can see, it's far more complicated than one might expect."

Dr Simon O'Toole from the Australian Astronomical Observatory was interviewed by Stuart Gary.

Comments (5)

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Lynne :

12 Aug 2013 5:12:26pm

I read Simon O'Toole's answer about measuring altitude on Mars with interest. In answering, he contrasted the Mars procedure with that use on earth - which he described as "...mean sea level is used as the reference point for measuring elevation" ..give or take. With global-warming-caused sea level rise (current and projected), does that mean that Mt Everest will be about 100 ft lower by 2100? ie do all altitudes have to be re-calculated? If so, at what point in time?

Dann :

Here in Australia, the Australian Height Datum (AHD) defines the zero point as the average sea level recorded at 30 tide gauges from around the continent for the years 1966 to 1968.

Sea levels have almost certainly changed since then, however the AHD zero point remains the same as it was when originally defined in 1971. The actual average sea level in Australia is no longer at 0.000m AHD, but we still use that historic zero point as the official 'sea level' for the purposes of measuring altitude.

Lynne :

14 Aug 2013 12:52:33pm

Thanks for that explanation, Dann. Makes sense to establish an arbitrary zero based on an agreed historical set of readings. But it does lead to an interesting paradox - for instance, Kiribati - elevation 2 m, but potentially beneath the real sea level 60 years hence (SMH 9/3/2012).